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Re: discard and v2 qcow2 images

From: Eric Blake
Subject: Re: discard and v2 qcow2 images
Date: Fri, 20 Mar 2020 14:35:44 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 3/20/20 1:58 PM, Alberto Garcia wrote:

when full_discard is false in discard_in_l2_slice() then the selected
cluster should be deallocated and it should read back as zeroes. This
is done by clearing the cluster offset field and setting OFLAG_ZERO in
the L2 entry.

This flag is however only supported when qcow_version >= 3. In older
images the cluster is simply deallocated, exposing any possible
previous data from the backing file.

Discard is advisory, and has no requirements that discarded data read back as zero. However, if write zeroes uses discard under the hood, then THAT usage must guarantee reading back as zero.

This can be trivially reproduced like this:

    qemu-img create -f qcow2 backing.img 64k
    qemu-io -c 'write -P 0xff 0 64k' backing.img
    qemu-img create -f qcow2 -o compat=0.10 -b backing.img top.img
    qemu-io -c 'write -P 0x01 0 64k' top.img

After this, top.img is filled with 0x01. Now we issue a discard

    qemu-io -c 'discard 0 64k' top.img

top.img should now read as zeroes, but instead you get the data from
the backing file (0xff). If top.img was created with compat=1.1
instead (the default) then it would read as zeroes after the discard.

I'd argue that this is undesirable behavior, but not a bug.

This seems like a bug to me, and I would simply forbid using discard
in this case (see below). The other user of full_discard = false is
qcow2_snapshot_create() but I think that one is safe and should be

--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3763,6 +3763,10 @@ static coroutine_fn int 
qcow2_co_pdiscard(BlockDriverState *bs,
      int ret;
      BDRVQcow2State *s = bs->opaque;
+ if (s->qcow_version < 3) {
+        return -ENOTSUP;
+    }

This changes it so you no longer see stale data, but doesn't change the fact that you don't read zeroes (just that your stale data is now from the current layer instead of the backing layer, since we did nothing at all).

I'm not opposed to the patch, per se, but am not convinced that this is a problem to worry about.

      if (!QEMU_IS_ALIGNED(offset | bytes, s->cluster_size)) {
          assert(bytes < s->cluster_size);
          /* Ignore partial clusters, except for the special case of the


Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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